1. Field of the Invention
This invention relates to a combined gas turbine plant comprising a combined gas turbine device which is adapted to serve as a prime mover and includes in combination a gas turbine, an exhaust heat recovery device and a steam turbine, the combined gas turbine plant being capable of being constituted as a combined gas turbine power plant or the like.
2. Description of the Prior Art
In general, a known type of combined gas turbine device comprises: an air compressor; a combustor for generating a gas of high temperature and high pressure by burning fuel while utilizing as an oxidizer air compressed by the air compressor; a gas turbine adapted to be operated by the gas of high temperature and high pressure generated by the combustor; an exhaust heat recovery device such as an exhaust heat recovery heat exchanger arranged at a location through which an exhaust gas passage for leading an exhaust gas from the gas turbine to the atmosphere extends, the exhaust heat recovery device generating stream of high temperature and high pressure by recovering exhaust heat from the exhaust gas from the gas turbine; and a steam turbine driven or rotated by the steam of high temperature and high pressure generated by the exhaust heat recovery device.
In recent years, combined gas turbine devices have been put into practical use, and a variety of combined gas turbine plants such as combined gas turbine power plants in which a combined gas turbine device is utilized as a prime mover have been constructed. In such combined gas turbine plants, there is a critical need to reduce the amount of NOx contained in the exhaust gases exhausted from the combined gas turbine device which is incorporated in the plant, in order to prevent the environmental pollution. Also, the plant is required to have good starting performance.
Conventionally, there has been known a combined gas turbine power plant in which a denitration device is installed in an exhaust heat recovery device for the purpose of reducing the amount of NOx contained in exhaust gas, and air extraction means is provided. During starting of the plant, i.e., until the time when the plant reaches an operating condition under a predetermined load, the air extraction means extracts a part of the air flowing from an air compressor to a combustion chamber of a combustor, and leads the extracted air into a portion of an exhaust duct of the gas turbine disposed on the upstream side, as viewed in an exhaust gas flow direction, of the exhaust heat recovery device. The extraction means is provided for preventing surging of the air compressor during starting of the plant, and for improving the ignitability and stability of flames by slightly increasing the fuel density in the combustor at a certain point of time during the starting process. However, the power plant of this type involves a problem in that the denitration efficiency of the denitration device deteriorates with the resultant increase in the amount of NOx contained in the exhaust gas. Because, the air introduced into the exhaust duct of the gas turbine by the extracting means is mixed with the exhaust gas from the gas turbine and hence lowers the temperature of the exhaust gas, and the exhaust gas of lowered temperature is introduced into the denitration device. Furthermore, since a long time is required for the denitration device to be heated up to a predetermined temperature by the exhaust gas, the length of time required for starting the plant becomes large.